A new numerical study has mapped the distribution and dynamics of tidal residual currents across China's marginal seas, from the Bohai Sea to the northern South China Sea. The research, published in the Journal of Xiamen University (Natural Science) in May 2026, provides a high-resolution view of these weak but persistent flows that can significantly influence coastal water exchange and the long-term transport of water, nutrients, and pollutants. The study was conducted by researchers from Xiamen University's College of Ocean and Earth Sciences and the 715th Research Institute of China State Shipbuilding Corporation Ltd.
Tidal residual currents arise because tidal motion does not average to zero everywhere over a tidal cycle, resulting in a net displacement of water parcels. While much weaker than instantaneous tidal currents, these residual flows can account for about 50–80% of the local flow between the Changjiang Estuary and the Subei Shoal, and may dominate in some shallow coastal areas. Previous studies in China focused mainly on Eulerian residual currents, which are time-averaged velocities at fixed locations and do not directly represent the net transport of water parcels. Lagrangian residual currents, calculated from the net displacement of water parcels over a tidal cycle, offer a more direct measure of material transport. However, systematic high-resolution simulations spanning China's marginal seas have been limited, and the relative effects of the Coriolis force, bottom friction, and velocity shear have not been fully resolved.
To address this gap, the team used the Regional Ocean Modeling System (ROMS) to simulate barotropic tidal motion over a domain spanning 99°E–150°E and 15°S–41°N at a horizontal resolution of 0.05°, with 50 vertical layers and 15 tidal constituents. The domain includes the Bohai Sea, Yellow Sea, East China Sea, northern South China Sea, and adjacent western North Pacific. Their analysis compared Eulerian residual currents, tidal Stokes drift, and Lagrangian residual currents to identify the mechanisms that govern them.
The simulations revealed distinct regional patterns. In the Bohai Sea, a large anticyclonic (clockwise) residual circulation dominates, with velocities of 0.5–3 cm/s, except in the northern Bohai Strait where velocities can reach 4–10 cm/s. The Yellow Sea contains several small cyclonic and anticyclonic residual circulations near the coast, while a southward residual current emerges from the Bohai Strait and extends along the central Yellow Sea. In the Taiwan Strait, residual currents flow predominantly northeastward, with a strong anticyclonic circulation around the Taiwan Bank.
The study also showed that tidal Stokes drift is comparable in magnitude to Eulerian residual currents in shallow waters but negligible in deep waters. Consequently, Lagrangian residual currents in shallow regions are directed more strongly toward the coast and are slightly faster than their Eulerian counterparts, whereas the two are nearly identical in deep waters. Bathymetric features such as coastlines, islands, shoals, and submarine ridges organize the residual-current field and generate numerous small-scale circulations. A residual-vorticity balance indicates that the interaction of bottom friction with velocity shear exerts the dominant control on the overall distribution of Eulerian residual currents. The bottom-friction term associated with water-depth gradients acts mainly in localized regions, while the Coriolis term influences the background residual vorticity and several regional structures.
These findings are relevant to coastal management and environmental protection because tidal residual currents contribute to the long-term transport and dispersion of pollutants, sediment, nutrients, and other suspended material. The results can inform coastal environmental assessment, marine engineering, channel maintenance, and the sustainable use of coastal resources. By clarifying where tidal residual currents are strongest and which mechanisms shape them, the study provides a physical basis for assessing long-term material transport across China's continental shelves. The research was supported by the National Natural Science Foundation of China (Grant No. 41776015) and the National Key Research and Development Program of China (Grant No. 2022YFF0801404). The full study is available at http://dx.doi.org/10.6043/j.issn.0438-0479.202412018.
